It has long been recognized that the ability of tumor tissues to metastasize constitutes a major portion of the life-threatening aspects of malignancy. Metastasis is the growth of secondary tumors at sites different from the primary tumor. Thus, despite surgical removal of the primary tumor, it may not be possible to arrest the progress of this condition. An understanding of the mechanism whereby metastasis occurs will be crucial to the development of protocols whereby the growth of secondary tumors can be controlled. In order to understand the mechanism of metastasis, it will be necessary to provide a model which permits identification of small numbers of tumor cells against a background of many host cells so that secondary tumor emboli and micrometastases can be observed over the course of real time.
Others have demonstrated extravasation and initial seeding steps in tumor metastasis in vitro using externally fluorescently labeled tumor cells. Khokha, R. et al., Cancer Metastasis Rev (1995) 14:279-301; Koop, S. et al., Cancer Res (1995) 55:2520-2523. Further, Margolis, L. B. et al., In Vitro Cell Dev Biol (1995) 31:221-226 was able to visualize the migration of externally fluorescently labeled lung tumor cells in host mouse lung in histoculture. In all cases, however, long-term observation was not possible due to the limitation of exogenous fluorescent labels. Retroviral transfer of a green fluorescent protein (GFP) gene has been shown to result in stable transfectants of human cancer cells in vitro. Levy, J. P. et al., Nature Biotechnol (1996) 14:610-614.
Attempts have been made to provide such a model using the .beta.-galactosidase gene as a marker (Lin, W. C. et al., Cancer Res (1990) 50:2808-2817; Lin, W. C. et al., Invasion and Metastasis (1992) 12:197-209). However, this marker has not proved satisfactory, as fresh or processed tissue cannot be used. The present invention provides a marker which permits visualization of tumor invasion and micrometastasis formation in viable fresh tissue.